Mucosal-associated invariant T (MAIT) cells reside primarily in the gut lamina propria and require commensal flora for selection/expansion. They are restricted by the highly conserved MHC class I-related molecule MR1 and, like most NK T cells, express an invariant TCRα chain. Although they probably contribute to gut immunity, MAIT cells have not been functionally characterized because they are so rare. To create a model in which they are more abundant, we generated transgenic mice expressing only the TCRα chain (Vα19i) that defines MAIT cells. By directly comparing Vα19i transgenic mice on MR1+/+ and MR1−/− backgrounds, we were able to distinguish and characterize a population of Vα19i T cells dependent on MR1 for development. MR1-restricted Vα19i transgenic T cells recapitulate what is known about MAIT cell development. Furthermore, a relatively high proportion of transgenic MAIT cells express NK1. 1, and most have a cell surface phenotype similar to that of Vα14i NK T cells. Finally, MR1-restricted Vα19i T cells secrete IFN-γ, IL-4, IL-5, and IL-10 following TCR ligation, and we provide evidence for what may be two functionally distinct MAIT cell populations. These data strongly support the idea that MAIT cells contribute to the innate immune response in the gut mucosa.